This invention relates to an improved pressure-sensitive adhesive. This adhesive is particularly useful with diaper closure tapes and the like, and more specifically, to a pressure-sensitive adhesive composition which provides improved performance characteristics when used in a diaper closure system having an adhesion surface treated with a release agent.
There are numerous patents and literature documents that are directed to the use of block copolymers and other elastomers in adhesive compositions which traditionally include the elastomer (e.g., a block copolymer such as an ABA block copolymer) and a tackifying resin. For example, U.S. Pat. No. 3,239,478 (Harlan), describes the use of an elastomeric based adhesive comprising at least 100 parts of an ABA block copolymer, a tackifying resin (25-300 parts) and an extender oil (5-200 parts). U.S. Pat. No. 3,242,110 (Korpman) describes pressure-sensitive adhesives formed from a broad group of elastomers including natural rubber and polyisoprene with a solid tackifier. The Korpman patent reports similar adhesion values for the natural rubber and the polyisoprene based adhesive.
As development of these elastomer based compositions has progressed it has been found that the adhesion properties of these adhesive compositions are extremely sensitive to the particular elastomer or elastomers used, and the proportion and type of modifiers such as solid or liquid tackifiers, aromatic or aliphatic tackifiers, plasticizers, extender oils, curing agents and the like. For example, Harlan alleged that its composition using a block copolymer and an oil achieved superior cohesive strength compared to traditional tackified natural rubber adhesives.
U.S. Pat. No. 3,681,190 (Dahlquist) describes a pressure-sensitive adhesive tape where the adhesive is substantially colorless and comprises a blend of an elastomeric polymer and an alkylated polystyrene tackifying resin. The colorless feature is attributed to the resin. Both natural rubber and polyisoprene-polystyrene based adhesive systems [Kraton.TM. 1107 admixed with a diblock copolymer], were exemplified. Significantly higher peel adhesions (ASTM D-1000) were reported for the block copolymer based systems.
U.S. Pat. No. 4,181,635 (Takamatsu et al.) describes a pressure-sensitive adhesive system formed from a wide range of elastomers, tackified with a low molecular weight polyisoprene (with a viscosity average molecular weight ranging from 8,000 to 77,000 with a molecular weight distribution between 1.0 and 2.7) and additional conventional tackifying resins. It is reported that polyisoprenes with high percentages of low molecular weight polyisoprene species yield adhesives that bleed and possibly lose tackiness at elevated temperatures (70.degree. C.). Too high a molecular weight polyisoprene reportedly yields inferior low temperature (e.g., 5.degree. C.) tack. The elastomers exemplified are masticated natural rubber and high molecular weight, cis-1,4-polyisoprene rubber.
Although there exists extensive art on the use of block copolymers in PSA compositions, due to the uncertainty in results and continuing need, the search continues for adhesive formulations with yet improved mixes of adhesion properties, such as shear strength and peel strength. Particularly, there is the continuing need for the identification of adhesive formulations that have a proper mix of adhesive properties such as would make them particularly suitable for use in closure systems such as for diapers.